<!DOCTYPE html>

<!--  -->
<html>
<head>
  <style>
    div,span {
      width: 100px;
      height: 40px;
      float:left;
      padding: 10px;
      margin: 10px;
      background-color: #EEEE00;
    }
  </style>
  <script src="http://code.jquery.com/jquery-latest.js"></script>
</head>

<body>
  <div class="notMe">div class="notMe"</div>
  <div class="myClass">div class="myClass"</div>
  <span class="myClass">span class="myClass"</span>

  <script>
    /* jQuery( ".class" )
        class: A class to search for. Un elemento puede tener múltiples clases; sólo una de ellas debe matechear.
    */
    $(".myClass").css("border","3px solid blue");
  </script>
 
  <div class="myclass">div class="notMe"</div>
  <div class="myclass otherclass">div class="myClass"</div>
  <span class="myclass otherclass">span class="myClass"</span>
  
  <script>
    /* más de una clase
    */
    $(".myclass.otherclass").css("border","13px solid red");
  </script>

</body>
</html>